Carbonate ion concentrations are important parameters in marine ecosystems. Investigations of the marine CO2 system are commonly conducted through measurements of four primary variables: total dissolved inorganic carbon (CT), total solution alkalinity (AT), CO2 fugacity (fCO2), and solution pH. Thermodynamic models link these four primary variables, whereby measurements of any two variables can be used to calculate the two remaining parameters (DOE, 1994,. Handbook of Methods, Version 2, A. G. Dickson & C. Goyet, eds. ORNL/CDIAC-74). These models also allow calculations of the concentrations of the individual forms of inorganic carbon in seawater: the dissolved concentrations of CO2 and H2CO2, and the free plus ion paired concentrations of bicarbonate, HCO3−, and carbonate, CO32−. Two of the directly measured and derived CO2 system variables can be highlighted because of their special significance in evaluations of global carbon fluxes and the biogeochemistry of marine carbonates in general. CO2 fugacity measurements are essential to descriptions of CO2 exchange at the air sea interface (DOE, 1994; McGillis, W. R. and Wanninkhof, R.; Mar. Chem. 2006, 100-108; Millero, F. J. Chem. Rev. 2007, 107, 308-341), and carbonate ion concentrations are essential to evaluations of (a) the mineralization rates of marine calcifiers (Langdon, C. and Atkinson, M. J., I Geophys. Res. 2005, 110, C09S05, doi: 10.1 029/2004JC002576) and (b) the dissolution rates of calcite and aragonite (CaCO3(S) polymorphs) both on the seafloor and in the water colunm (Keir, R. S. Geochim. Cosmochim. Acta. 1980, 44, 241-252; Morse, J. W. Amer. J. Sci. 1978, 278, 344-355; Acker, J. G., et al., Geochim. et Cosmochim. Acta 1987, 51, 2171-2175). Rising atmospheric carbon dioxide concentrations over the past two centuries have led to increasing CO2 uptake by the oceans (Royal Society, 2005, Ocean Acidification due to Increasing Atmospheric Carbon Dioxide. Policy Document 12/05, The Royal Society). This process, which is decreasing the pH of the upper ocean, is reducing oceanic carbonate ion concentrations and thus the level of saturation of calcium carbonate (Broecker, W. S., et al., Science 1979, 206, 409-418; Feely, R. A., et al., Science. 2004, 305, 362-366; Orr, J. C., et al., Nature. 2005, 437, 681-686.). If the trend continues, it will have a seriously negative impact on key marine organisms such as corals and some plankton (Kleypas et al., Impacts of Acidification on Coral Reefs and Other Marine Calcifiers: A guide for Future Research. Report of a workshop held 18-20 Apr. 2005, St. Petersburg Fla., sponsored by NSF, NOAA and U.S. Geological Survey. 88pp.). In view of the importance of carbonate ion concentrations ([CO32−]T) to the oceans' rapidly evolving carbonate system, it is then highly desirable to move ([CO32−]T from the rank of derived CO2 system variables to the list of primary measured variables. The present invention provides methods and associated systems to meet this important objective.